Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

No annotations are provided, so the description carries the full burden of behavioral disclosure. It only states the subject matter (supported countries and E.164 hints) but does not explicitly say whether the tool is read-only, what the response shape is, or if any prerequisites or side effects exist. This leaves notable gaps for a zero-parameter tool.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

The description is a single, concise sentence that immediately communicates the tool's purpose without any filler or redundant information. It is appropriately sized for the tool's simplicity.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

Given the tool's low complexity and zero parameters, the description gives a basic idea of what to expect, but since there is no output schema, it fails to specify the return format (e.g., list of country names, ISO codes, dialing prefixes). This is a notable gap for a reference tool.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

The tool has zero parameters and an empty input schema. The description correctly implies that no additional input is required, matching the schema. With 0 parameters, the baseline is 4, and the description adds no unnecessary parameter details.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

The description clearly identifies the tool as providing supported fax destination countries and E.164 number hints. It uses a specific resource ('Fax-Zielländer') and differentiates itself from sibling tools by focusing on country compatibility and number formatting rather than operations like fax_quote or fax_status.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

The description implies that this tool should be used when you need to know which countries are supported for fax or how to format numbers per E.164. However, it does not explicitly state when to use this versus alternatives (e.g., list_providers), and lacks any exclusionary guidance.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.